This invention relates to a method and apparatus to detect the presence of water on a surface.
The method and apparatus may be used to detect the presence of water on an inside surface of a fuel gas main or pipe to indicate ingress of water into the main or pipe.
According to a first aspect of the invention a method of detecting the presence of water on a surface comprises emitting an optical signal comprising at least a first wavelength and a second wavelength, both said wavelengths being absorbed by water but said first wavelength being absorbed to a greater extent than said second wavelength so that said second wavelength provides a reference, directing said optical signal onto a said surface from which the signal is reflected, alternately passing the reflected signal through a first optical filter which passes substantially only an optical signal of said first wavelength and through a second optical filter which passes substantially only an optical signal of said second wavelength, and observing when the strength of a signal emergent from the first optical filter differs in a pre-determined way from the strength of the reference passed by the second optical filter to indicate the presence of water on the surface.
According to a second aspect of the invention apparatus to detect the presence of water on a surface comprises an optical source to emit an optical signal comprising at least a first wavelength and a second wavelength, both said wavelengths being absorbed by water but said first wavelength being absorbed to a greater extent than said second wavelength so that said second wavelength provides a reference, means to direct said optical signal onto a said surface for reflection thereby, a first optical filter which passes only an optical signal of said first wavelength, a second optical filter which passes only an optical signal of said second wavelength, means to interpose alternately the first optical filter and the second optical filter in the path of the reflected optical signal, and means to observe when the strength of a signal emergent from the first optical filter differs in a predetermined way from the strength of the reference passed by the second optical filter to indicate the presence of water on the surface.